CN102626778A - Method and device for preparing semi-solid alloy slurry by applying square wave pulse current - Google Patents

Abstract

A method and device for preparing a semi-solid alloy slurry by applying a square wave pulse current. The subsequent steps of the method include: (1) adjusting the refined melt in the crucible to within 40°C to 70°C above the liquidus line, and then cooling to 10-20°C above the liquidus line, let it stand for 15 minutes; (2) Select and control the cooling rate of the melt according to the type of alloy, when the melt is cooled to 5°C above the liquidus line, apply a square wave to the melt Pulse current, when the melt continues to cool to 5°C below the liquidus line, stop the application to ensure that the applied temperature range of the square wave pulse current covers the entire nucleation stage of the melt; (3) The melt after treatment is slowly cooled to The semi-solid forming temperature is within ±5°C to make a semi-solid slurry. The device includes a square wave pulse power supply, a current applying device and a heating and heat preservation device. The invention applies square wave pulse current treatment to the melt in the nucleation stage and controls the cooling rate to obtain the semi-solid alloy slurry whose primary phase is spherical or granular, and is suitable for the preparation of semi-solid slurry of various alloys.

Description

Method and device for preparing semi-solid alloy slurry by applying square wave pulse current

technical field

The invention relates to a metal preparation method and device, in particular to a method and device for preparing semi-solid alloy slurry by applying a square wave pulse current suitable for the semi-solid forming process of metal materials, especially suitable for magnesium alloy and aluminum alloy semi-solid The forming process belongs to the technical field of metal materials and metallurgy.

Background technique

Semi-solid processing technology is a new method of metal forming proposed by Professor M.C.Flemings of the Massachusetts Institute of Technology in the 1970s. This method uses the process of metal transition from liquid to solid or from solid to liquid (that is, liquid-solid coexistence) The characteristics possessed by it are formed. This new forming processing method combines the strengths of solidification processing and plastic processing, that is, the processing temperature is lower than that of the liquid state, and the deformation resistance is smaller than that of the solid state. Known as "the new generation of metal forming technology in the 21st century". The application of semi-solid forming technology in aluminum and magnesium alloys effectively reduces the processing temperature, reduces defects such as pores and shrinkage cavities, improves the yield and product quality of parts, and promotes the development of metal forming methods.

According to different technological processes, semi-solid forming processes are divided into thixotropic forming and rheological forming. The former refers to solidifying the alloy melt into a billet, dividing the billet into a certain size as required, and reheating to the semi-solid area of the metal for processing. The thixoforming process is long and the production capacity is low, but the product quality is easy to control; The latter means that after the semi-solid rheological slurry is obtained, the forming process is performed directly, the process flow is short, and the production capacity of the enterprise can be significantly improved, so it has a broader application prospect in industrial production. In recent years, the research focus of semi-solid forming technology has focused on rheological forming. The rheological forming process first needs to prepare high-quality semi-solid alloy slurry. The traditional preparation methods of semi-solid alloy slurry include mechanical stirring method and electromagnetic stirring method, and the semi-solid slurry is mainly obtained by crushing dendrites through vigorous stirring. Some of these methods are easy to cause melt pollution, some have complex equipment, large investment, and some have poor operability and are difficult to control, so that their application is restricted to varying degrees.

Since the 1990s, the research idea of semi-solid pulping has changed from smashing dendrites through vigorous stirring to promoting nucleation and controlling growth, that is, firstly promoting a large number of nucleation in the melt to obtain a sufficient number of free crystals, and then The morphology of the primary phase was improved during the slow cooling process, and finally a semi-solid slurry with a spherical primary phase was obtained. Using this new idea, researchers have developed new methods for semi-solid pulp preparation such as SSR (Semisolid rheocasting), NRC (New rheocasting), LSPSF (Low superheat pouring with a shear field), and these methods generally have the advantages of simple equipment and convenient operation. , The advantage of low cost.

In the semi-solid pulping idea of promoting nucleation and controlling growth, the first condition is to promote a large number of nucleation in the melt. Existing studies have shown that applying pulse current to the melt during conventional solidification can increase the nucleation rate; at the same time, the magnetostrictive effect and shear stress generated by the current can break dendrites, spheroidize grains, and inhibit growth. , Reduce segregation. The effect of pulse current to promote nucleation is remarkable, and it is easy to operate, low in energy consumption, and does not pollute the melt. It is a potential new method for preparing semi-solid slurry. If pulse current can be applied in industry, it will greatly promote the development of semi-solid slurry. The development of forming technology.

At present, the research on the preparation of semi-solid slurry by applying pulse current is in its infancy. After literature search, it was found that Chinese Patent No. 200410087675.0 discloses a low-voltage pulsed electric field preparation method and special equipment for a non-dendritic semi-solid alloy. In this patent, a low-voltage pulse electric field is applied to the alloy melt during the solidification process. Under the appropriate pulse current process parameters, the primary phase in the solidification structure changes from a developed dendrite to a spherical or near-spherical shape. However, there are still some deficiencies in this patent: firstly, the used pulse power supply is an RLC circuit. In the RLC circuit, the current is realized by the charging and discharging of the capacitor, and the pulse current density of the alloy liquid is very different at each moment. Quantitative research on the solidification process brings great difficulties; secondly, the RLC pulse power supply is huge in size, requires a special site, and lacks flexibility, which brings a lot of inconvenience to production and experiments; The bottom end of the crucible needs to be inserted, and the lower end electrode needs to pass through the bottom of the crucible, which is very difficult in actual operation. In addition, J.H.Ma et al. published Grain refinement of pure Al with different electric current pulse modes (refining effect of different pulse current modes on pure aluminum) in "Materials Letters" 2009, No. 63, pp. 142-144, which studied different electrode positions For the grain refinement effect of pure aluminum, the results show that the grain refinement effect obtained by the parallel electrode mode is better than that of the upper and lower electrode mode.

Contents of the invention

The purpose of the present invention is to provide a method for preparing semi-solid alloy slurry by applying a square wave pulse current with wide application range and low cost, and at the same time provide a device for realizing the method. In the nucleation stage of the alloy solidification process, the present invention applies a square wave pulse current to the melt, utilizes the interaction of the square wave pulse current on the Joule heating effect and electromagnetic oscillation effect of the alloy, improves the nucleation rate of the melt, and controls Slower cooling rate further inhibits dendrite growth to obtain spherical or granular primary phases.

To achieve the above object, the present invention solves its technical problems through the following technical solutions:

A method for applying a square wave pulse current to prepare a semi-solid alloy slurry, comprising the previous conventional melting, refining, slag removal and degassing processes, and its follow-up process includes the following steps:

(1) Adjust the refined alloy melt in the crucible to within 40°C to 70°C above the liquidus line, then cool the alloy melt to 10°C to 20°C above the liquidus line, and let it stand for 15 minutes;

(2) According to the type of alloy, select and control the cooling rate of the alloy melt. When the alloy melt is cooled to 5°C above the liquidus line, apply a square wave pulse current to the alloy melt. When the alloy melt continues to cool to the liquid phase When the temperature is 5°C below the line, stop applying the square wave pulse current to ensure that the temperature range for applying the square wave pulse current covers the entire nucleation stage of the alloy melt;

(3) Slowly cool down the alloy melt treated with the square wave pulse current to within the range of the semi-solid forming temperature ±5° C. to prepare a semi-solid alloy slurry.

In the method of the present invention, the square wave pulse current is 20-600A, the pulse frequency is 0.5-500Hz, the duty cycle is 15-100%, and the cooling rate is 0.5-2°C/min.

Another technical solution of the present invention is:

A device for implementing the method described in claim 1 by applying a square wave pulse current to prepare a semi-solid alloy slurry, which includes a square wave pulse power supply, a current application device and a heating and heat preservation device, and the current application device is arranged on the heating and heat preservation device , and connected with the square wave pulse power supply, the square wave pulse current generated by the square wave pulse power supply acts on the alloy melt in the heat preservation device through the current applying device.

The square-wave pulse power supply includes a power frequency rectifier, an inverter, an intermediate frequency transformer, an intermediate frequency rectifier and a Hall device connected in sequence, and the three-phase power frequency alternating current is rectified by the power frequency rectifier to become direct current, and then transmitted to the inverter for transformation It is converted into an intermediate frequency alternating current, then stepped down by an intermediate frequency transformer, rectified by an intermediate frequency rectifier, and then filtered by a Hall device, and then output. The circuit adopts current negative feedback control.

The current applying device includes an electrode frame, an electrode clamp, a lock nut and an electrode, the electrode frame is fixed on the heating and heat preservation device, the electrode clamp is adjustablely fixed on the electrode frame through the lock nut, and It is connected with the pulse power supply through a cable, and the electrode is arranged in the electrode holder and extends vertically into the alloy melt.

The heating and heat preservation device includes a thermocouple, a resistance furnace, a temperature control cabinet and a crucible. The crucible is placed in the resistance furnace, and the thermocouple extends into the alloy melt and is connected with the temperature control cabinet.

Since the square wave pulse current density is relatively stable, and the frequency domain is wide, the energy attenuation of high-order harmonics is slow, so the promotion of nucleation is significant in the process of semi-solid pulping, and the amplitude of the electromagnetic oscillation induced in the alloy melt , The frequency response is obvious, so the method for preparing the semi-solid alloy slurry of the present invention has the following advantages:

(1) The effect of square wave pulse current is obvious, and it is applicable to a wide range of alloys.

(2) The process is simple, no stirring is required, no pollution, the melt pollution is small during the treatment process, and the obtained slurry is of high quality.

(3) Since the electrode insertion method is chosen to be inserted parallel to the liquid surface, it can not only obtain the best effect, but also achieve the beneficial effect of convenient operation.

(4) It can not only handle a small amount of melt, but also meet the needs of large-scale pulping, which improves the adaptability to both experiment and production.

(5) The equipment is simple, the energy consumption is small, the cost is low, and there is no special requirement for the site.

Description of drawings

Fig. 1 is a structural schematic diagram of the present invention.

Figure 2 is a schematic diagram of the working principle of the square wave pulse power supply.

Figure 3 is a comparison diagram of the semi-solid structure of AZ91D magnesium alloy before and after square wave pulse current treatment, Figure 3(a) is the original structure without treatment, and Figure 3(b) is the typical structure after square wave pulse current treatment.

Figure 4 is a comparison diagram of the ZL101 aluminum alloy semi-solid structure before and after square wave pulse current treatment. Figure 4(a) is the original structure without treatment, and Figure 4(b) is the typical structure after square wave pulse current treatment.

Figure 5 is a comparison diagram of the semi-solid structure of A390 aluminum alloy before and after square wave pulse current treatment, Figure 5(a) is the original structure without treatment, and Figure 5(b) is the typical structure after square wave pulse current treatment.

In the above figure, 1-square wave pulse power supply, 2-electrode frame, 3-electrode chuck, 4-lock nut, 5-electrode, 6-thermocouple, 7-resistance furnace, 8-temperature control cabinet, 9- Crucible, 10-melt.

Detailed ways

The embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. This embodiment provides detailed implementation methods and specific operating procedures on the premise of the technical solution of the present invention, but the protection scope of the present invention is not limited to the following embodiments. .

The main idea of the present invention is: in the solidification process of the alloy, mainly in the nucleation stage, a square wave pulse current is applied to the melt, and the interaction of the square wave pulse current on the Joule heating effect and the electromagnetic oscillation effect of the alloy is used to improve the temperature of the melt. The nucleation rate and controlled slower cooling rate further inhibit dendrite growth to obtain spherical or granular primary phases.

As shown in FIG. 1 , the device for applying square wave pulse current to prepare semi-solid alloy slurry according to the present invention includes a square wave pulse power supply 1 , a current applying device and a heating and heat preservation device. Wherein, the current applying device is arranged on the heating and holding device, and is connected with the square wave pulse power supply 1, and the square wave pulse current generated by the square wave pulse power supply 1 acts on the alloy melt in the heating and holding device through the current applying device. .

The square wave pulse power supply 1 is used to provide square wave pulse current, which includes a power frequency rectifier, an inverter, an intermediate frequency transformer, an intermediate frequency rectifier and a Hall device connected in sequence. As shown in Figure 2, when working, the three-phase 380V power frequency (50Hz) alternating current is rectified by the power frequency rectifier and becomes direct current, and then the direct current is transmitted to the inverter, so that the direct current is converted into intermediate frequency alternating current (about 20KHz), and then passed through the intermediate frequency The transformer is stepped down, the intermediate frequency rectifier is rectified, and the output is filtered by the Hall device. The current negative feedback control technology is used in the circuit to ensure the stability of the output current. Starting the square wave pulse power supply 1 can output square wave pulse current.

The function of the current applying device is to connect the heating and heat preservation device and the square wave pulse power supply 1 , and the current applying device includes an electrode frame 2 , an electrode chuck 3 , a locking nut 4 and an electrode 5 . The electrode frame 2 is fixed on the heating and heat preservation device to play a supporting role, and it is welded by stainless steel. The electrode clamp 3 is fixed on the electrode frame 2 with a lock nut 4, and its position is adjustable so as to adjust the distance between the two electrode clamps 3, which are connected to the pulse power supply 1 by cables. The electrode 5 is a stainless steel electrode, and the electrode 5 is arranged in the electrode holder 3 and vertically extends into the alloy melt 10 .

The heating and heat preservation device includes a thermocouple 6 , a resistance furnace 7 , a temperature control cabinet 8 and a crucible 9 . The alloy melt 10 is placed in the crucible 9 and melted therein. The crucible 9 is placed in the resistance furnace 7. The thermocouple 6 extends into the alloy melt 10 and is connected with the temperature control cabinet 8. The thermocouple 6 and The temperature control cabinet 8 measures and controls the temperature and cooling rate of the alloy melt 10 . The function of the heating and holding device is to melt the alloy melt 10, keep it above the liquidus line for heat preservation, control the cooling rate of the alloy melt 10, and ensure that the square wave pulse current application interval covers the entire nucleation stage.

The method for preparing a semi-solid alloy slurry by applying a square wave pulse current with the above-mentioned device includes the previous conventional melting, refining, slag removal and degassing processes, and the subsequent process includes the following steps:

(1) Adjust the refined alloy melt in the crucible to within 40°C to 70°C above the liquidus line, then cool the alloy melt to 10°C to 20°C above the liquidus line, and keep it for 15 minutes to promote the alloying process. Uniform distribution of temperature field and solute field inside the melt 10.

(2) Select and control the cooling rate of the alloy melt 10 according to the type of alloy; before applying the current, use the electrode holder 2 to vertically insert the electrode 5 into the alloy melt 10, and place the two electrodes 5 in the diameter direction of the crucible 9 respectively The two ends of the electrode 5 are not in contact with the crucible wall or the bottom of the crucible, and the upper end of the electrode 5 is connected to the positive and negative poles of the square wave pulse power supply 1; when the alloy melt 10 is cooled to 5°C above the liquidus line, start the square wave pulse power supply 1. Apply a square wave pulse current to the alloy melt 10. When the alloy melt 10 continues to cool to 5°C below the liquidus line, turn off the power and stop applying the square wave pulse current to ensure that the temperature range of the square wave pulse current is applied to cover the alloy The entire nucleation stage of the melt 10; according to the different types and components of the prepared alloy, different process parameters for applying square wave pulse current are determined. The range is: square wave pulse current is 20-600A, pulse frequency is 0.5-500Hz, accounting The air ratio is 15-100%, and the cooling rate is 0.5-2°C/min. Due to the different cooling rates, the application time of the pulse current is generally 5 to 20 minutes.

(3) Slowly cool down the alloy melt treated with the square wave pulse current to within the range of the semi-solid forming temperature ±5° C. to prepare a semi-solid alloy slurry.

The following are examples of preparation of semi-solid alloy slurry using the method of the present invention.

Example 1

The liquidus temperature of AZ91D magnesium alloy is 595℃, and the solidus temperature is 470℃. The AZ91D alloy is melted and refined in the electric resistance furnace 7, and the whole process is protected with a mixed gas of SF ₆ and CO ₂ to prevent oxidation and combustion. The temperature field and the solute field of the melt 10 are homogenized, the electrode 5 is inserted into the alloy melt 10 , and the electrode clamp 3 is fixed with the lock nut 4 . When the alloy melt 10 is cooled to 600°C, start the square wave pulse power supply 1 to start processing the alloy melt 10, the current is 20A, the pulse frequency is 0.5Hz, and the duty cycle is 100%, and the processing continues until the temperature of the alloy melt 10 The temperature is lowered to 590°C, during which the average cooling rate of the alloy melt 10 is controlled to be 0.5°C/min. After the pulse current is applied, the electrode 5 is taken out, the electrode holder 2 is removed, and the alloy melt 10 is slowly cooled to a predetermined temperature to obtain a semi-solid slurry. The semi-solid microstructures of AZ91D magnesium alloy before and after square wave pulse current treatment are shown in Fig. 3(a) and (b). It can be seen from the figure that after being treated with a square wave pulse current, the primary phase in the AZ91D semi-solid structure changes from coarse dendrites to spherical or granular equiaxed crystals.

Example 2

The liquidus temperature of ZL101 aluminum alloy is 586°C, and the solidus temperature is 516°C. The ZL101 alloy is melted and refined in the resistance furnace 7, and after the slag removal and degassing process, the temperature is lowered to 600°C and kept for 15 minutes, and the electrode 5 is inserted into the alloy melt 10, and the electrode is fixed with the lock nut 4 Chuck 3. When the alloy melt 10 is cooled to 591° C., start the square wave pulse power supply 1 to start processing the alloy melt 10. The current is 600 A, the pulse frequency is 500 Hz, and the duty ratio is 15%. The temperature of the alloy melt 10 continues to drop. During this period, the average cooling rate of the alloy melt 10 was controlled to be 2°C/min. After applying the pulse current, the electrode 5 is taken out, the electrode frame 2 is removed, and the alloy melt 10 is slowly cooled to a predetermined temperature to obtain a semi-solid slurry. The semi-solid microstructure of ZL101 aluminum alloy before and after square wave pulse current treatment is shown in Fig. 4(a) and (b). It can be seen from the figure that the primary phase in the ZL101 semi-solid structure changes from thick dendrites to spherical equiaxed crystals after being treated with square wave pulse current.

Example 3

The liquidus temperature of A390 aluminum alloy is 616°C, and the solidus temperature is 568°C. The A390 alloy is melted and refined in the resistance furnace 7, refined with hexafluoroethane at 720°C, and after slag removal and degassing processes, the temperature is lowered to 630°C and kept for 15 minutes, and the electrode 5 is inserted into the alloy melt 10 , use the lock nut 4 to fix the electrode holder 3. When the alloy melt 10 is cooled to 621°C, start the square wave pulse power supply 1 to start processing the alloy melt 10, the current is 300A, the pulse frequency is 200Hz, and the duty cycle is 50%, and the temperature of the alloy melt 10 continues to drop. during which the average cooling rate of alloy melt 10 is controlled to be 1°C/min. After the pulse current is applied, the electrode 5 is taken out, the electrode holder 2 is removed, and the alloy melt 10 is slowly cooled to a predetermined temperature to obtain a semi-solid slurry. The semi-solid microstructure of A390 aluminum alloy before and after square wave pulse current treatment is shown in Fig. 5(a) and (b). It can be seen from the figure that the primary phase in the A390 semi-solid structure changes from thick dendrites to spherical equiaxed crystals after being treated with square wave pulse current.

The semi-solid slurry prepared by the method for preparing semi-solid alloy slurry by applying a square wave pulse current in the present invention can be directly used for semi-solid die-casting, extrusion or rolling.

Claims (6)

1. one kind applies the method that square wave pulsed current prepares semi-solid alloy slurry, comprises preceding road conventional fusing, refining, skims and the degasification operation, and it is characterized in that: the follow-up operation of said method may further comprise the steps:

(1) alloy melt that obtains through refining in the crucible is adjusted to more than the liquidus curve in 40 ℃～70 ℃, the cooled alloy melt leaves standstill insulation 15 minutes to more than the liquidus curve 10～20 ℃ then;

(2) according to alloy species; Select and control alloy melt cooldown rate;, alloy melt 5 ℃ the time, alloy melt is applied square wave pulsed current more than being cooled to liquidus curve, when alloy melt continues to be cooled to below the liquidus curve 5 ℃; Stop to apply square wave pulsed current, guarantee to apply the whole forming core stage of the temperature range covering alloy melt of square wave pulsed current;

(3) alloy melt after will passing through square wave pulsed current and handling slowly is cooled within the scope of semi-solid-state shaping temperature ± 5 ℃, processes semi-solid alloy slurry.

2. according to claim 1ly apply the method that square wave pulsed current prepares semi-solid alloy slurry; It is characterized in that: square wave pulsed current is 20～600A; Pulse frequency is 0.5～500Hz, and dutycycle is 15～100%, and cooldown rate is 0.5～2 ℃/min.

3. device that square wave pulsed current prepares semi-solid alloy slurry that applies of implementing the said method of claim 1; It is characterized in that: said device comprises square-wave pulse power source, electric current bringing device and heating and heat-insulating device; This electric current bringing device is arranged on the heating and heat-insulating device; And be connected with square-wave pulse power source, the square wave pulsed current that this square-wave pulse power source produces acts on the alloy melt in the heating and heat-insulating device through the electric current bringing device.

4. according to claim 3ly apply the device that square wave pulsed current prepares semi-solid alloy slurry; It is characterized in that: said square-wave pulse power source comprises frequency rectifier, inverter, intermediate-frequency transformer, intermediate frequency rectifier and the hall device that connects successively; Become direct current after the rectification of three-phase main-frequency alternating current process frequency rectifier; Be transferred to inverter again and be transformed into medium frequency alternating current; Then through behind intermediate-frequency transformer step-down, the intermediate frequency rectifier rectification by exporting after the hall device filtering, adopt Current Negative Three-Point Capacitance control in the circuit.

5. according to claim 3ly apply the device that square wave pulsed current prepares semi-solid alloy slurry; It is characterized in that: said electric current bringing device comprises arc-spark stand, terminal clamp, locking nut and electrode; Said arc-spark stand is fixed on the heating and heat-insulating device; Said terminal clamp is fixed on the arc-spark stand through locking nut adjustable positions ground, and is connected through cable with the pulse power, and said electrode is arranged in the terminal clamp and vertically stretches into alloy melt.

6. according to claim 3ly apply the device that square wave pulsed current prepares semi-solid alloy slurry; It is characterized in that: said heating and heat-insulating device comprises thermocouple, resistance furnace, temperature control cabinet and crucible; Said crucible is positioned in the resistance furnace, and said thermocouple stretches into alloy melt and is connected with the temperature control cabinet.

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